Pressure-sensitive recording sheets utilizing the color forming reaction of electron donating color formers and electron accepting color developers are widely used as pressure-sensitive copying paper and do not require the use of carbon paper. Such pressure-sensitive recording sheets are described in, for example, U.S. Pat. Nos. 2,711,375, 2,712,507, 2,730,456, 2,730,457, and 3,617,334, and Japanese Patent Publication Nos. 18317/63, 1178/72, and 20972/72.
The most typical pressure-sensitive recording sheet is as follows:
An electron donating color former is dissolved in a suitable solvent, microencapsulated, and coated on a substrate of the sheets to produce an "upper sheet". Further, an electron accepting color developer is coated on a substrate of the sheets to produce a "lower sheet". Moreover, the above-prepared microcapsules are coated on one surface of a substrate of the sheets, and the color developer on the other surface to produce an "intermediate sheet". The upper sheet is superposed on the lower sheet in such a manner that the coated surfaces face each other, or alternatively, when it is desired to produce a number of copies, one intermediate sheet or a plurality of intermediate sheets are interposed between the upper and lower sheets. On application of pressure on the above-superposed sheets, i.e., a pressure-sensitive copying paper, by typing or writing, the microcapsules existing at pressure-applied areas are broken, allowing the color former contained therein to go out and move to the color developer layer. Thus, the color former reacts with the color developer, producing a color image pattern.
These pressure-sensitive recording sheets are widely used, for example, as chits or printing papers for a computer, and their usefulness is very high. Thus, various improvements in respect of quality have been increasingly demanded to be added to the pressure-sensitive recording sheets.
In recent years, when printing with a pressure-sensitive sensitive recording sheet or pressure-sensitive copying paper, not only the color developer-coated surface, but also the microcapsule-coated surface is often printed on. In this case, problems arise, particularly when the intermediate sheet is printed on; i.e., when the microcapsule-coated surface or the color developer-coated surface is printed on, the microcapsules are broken by application of pressure. As a result of this pressure the oil contained in the microcapsules comes out and permeates the base paper, and finally reaches the color developer-coated surface at the opposite side, forming a color and causing the so-called fog. In particular, when the metal salts of aromatic carboxylic acids are used as color developers, the phenomenon of fog easily occurs because of their high color-forming properties.
In order to eliminate the problem of fog, various methods have been developed. For example, in one method pressure-protecting agents, such as starch particles, are added to the microcapsule layer, in another method the size of microcapsules is decreased, in still another method the amount of a binder added is increased, and in yet another method the amount of the color former oil being added is decreased. Although these methods are now in commercial use they are somewhat undesirably in that they reduce the color density of the pressure-sensitive recording sheet.
In addition, a method of sizing the surface of the base paper using water-soluble polymers, such as a starch solution, polyvinyl alcohol (PVA), modified PVA, sodium alginate, gelatin, carboxymethyl cellulose (CMC), hydroxyethyl cellulose (HEC), and styrene-butadiene rubber (SBR), is in commercial use. In accordance with this method, however, no sufficient effect can be obtained unless the amount of the water-soluble polymer being used is increased. This is disadvantageous from a viewpoint of cost, and furthermore, reduces the color-forming properties.
Moreover, a method is known which comprises treating a base paper with specific fluoro compounds, such as a chromium complex salt of perfluoromonocarboxylic acid (Japanese Patent Application (OPI) No. 98913/73 (the term "OPI" as used herein means a "published unexamined Japanese patent application")) and perfluoroalkylsulfonamides (Japanese Patent Application (OPI) No. 125019/77) to make it oil-resistant. This method, however, seriously increases the cost of the base paper and, therefore, some difficulty is encountered in the industrial practice of the method.